In attempts to increase the flame resistance of normally flammable thermoplastic resins, it has been a common practice to blend monomeric phosphites, phosphoric acid esters, thiophosphoric acid esters, containing halogenated organic alkyl radicals and halogenated organic compounds into the carbonate polymer. Increasing flame resistance becomes even more problematic when a rubber component is incorporated into the blends to increase low temperature impact resistance. The rubber modified thermoplastic resins frequently exhibit substantially poorer flame resistance properties than do unmodified thermoplastic resins.
Accordingly, there exists a need for rubber modified thermoplastic resins which exhibit a high resistance to flammability. Such a rubber modified thermoplastic resin would satisfy a long felt need, expanding the possible applications of rubber modified thermoplastic resins and thus constitute a notable advance in the state of the art.
Henton, in U.S. Pat. No. 4,619,968 describes carbonate polymer blends exhibiting flame resistant properties comprising a carbonate polymer grafted rubber compound, such as acrylonitrile/butadiene/styrene, in which at least a portion of the grafted on the rubber backbone is a flame resistant polymer; such as bromostyrene. The composition is claimed to achieve both flame and sharp notch impact resistance. However, the patentee does not teach the use of a silicone-based modifier with vinyl-based polymer grafts capable of exhibiting flame retardant properties.
There have been many attempts in the art to provide polyorganosiloxane-based graft copolymers which may be useful as impact strength modifiers for thermoplastic resins. See, for example, U.S. Pat. No. 2,891,920 (J. F. Hyde, et al.); and O. Graiver, et al., Rubber Chem. Tech., 56 (5), 918 (1983).
U.S. Pat. No. 3,898,300 states that a polyorganosiloxane-based graft copolymer for improving the impact strength of S/AN resin is formed by grafting S/AN co-monomers in an emulsion system onto the vinylsiloxane or allylsiloxane containing silicone substrate. U.S. Pat. No. 4,071,577 describes a similar approach by using a mercaptosiloxane in place of vinyl-group containing siloxanes. European Patent No. 0,166,900 reports further improvement of polysiloxane-based graft polymers and increased S/AN impact strength by using acryloxy-functionalized siloxane as the graft-linking agent. These graft polymers are utilized in connection with the impact modification of S/AN. British Patent No. 1,590,549 describes the use of a polyorganosiloxane graft copolymer in various plastic molding compositions. Similarly, European Patent Application No. 0,249,964 describes the use of a polyorganosiloxane graft copolymer in the polycarbonate containing blends
None of the references disclose the in-situ co-homopolymerization of vinyl monomer in the presence of siloxanes in an emulsion system, as described hereinbelow. The present invention is also directed to the use of graft polymers provided by subsequent graft polymerization of vinyl monomers (e.g. polymethyl (meth)acrylate, polystyrene or styrene/acrylonitrile copolymer) in the presence of such a co-homopolymerized polyorganosiloxane/vinyl-based substrate. Furthermore none of the prior art references have been shown to provide flame resistance by grafting onto the organosiloxane-based substrates a vinyl-based polymer capable of exhibiting flame retardant properties.
Unexpectedly, use of vinyl-based polymers capable of exhibiting flame retardant properties in the subsequent stages of the graft polymer, produces a novel flame retardant useful in imparting flame resistant properties on normally flammable thermoplastic resins and concurrently increasing both notched impact resistance and low temperature ductility.